Methods are generally known in multifarious variations, it being possible in principle to distinguish between 2D and 3D reconstruction methods, and these methods are used in essence for patient diagnostics. Because of the ionizing property of the radiation used for the purpose of scanning the patient, and of the risk accompanying it with reference to cell generation, the concern when using these methods is always to carry out the examinations at the lowest possible dose. Owing to this low available dose as the patient is being scanned, the quantum noise present becomes highly relevant to image quality and impairs the image quality.
In order to improve the image quality, it is proposed in part to use nonlinear image filters that, by contrast with simple linear methods, can reduce the noise without impairing edges of objects included in the image. Usually, such intelligent filters use the image properties in the two- or three-dimensional environment of the respective pixel in order, firstly, to determine the position of edges and, subsequently to apply suitable filters, for example in a fashion parallel to detected edges.
Such a method is described, for example, in patent application DE 10 2004 008 979.5-53 from Siemens. Such a method for noise reduction has, however, the following disadvantage:
If the filter is executed in only one step, the spatial range must be sufficiently large in order to implement an adequate low-pass action. However, this raises the risk of impairing small structures. If it is desired to keep the range of the filters as small as possible and yet to reduce the noise effectively, recourse must be made to iterative methods that are problematical in practical use with regard to performance and because of the restricted computational capacity.
Another possibility for reducing quantum noise is described, for example, in laid-open patent application DE 103 05 221 A1, the entire contents of which are hereby incorporated herein by reference. The disclosure content of this document is hereby incorporated fully into the present patent application. This document exhibits a method for reducing noise structures in two- or three-dimensional images, the same object being recorded under identical geometrical conditions or ones that are changed in a defined way, a transformation of the images produced subsequently being carried out in a frequency domain, and frequency-dependent correlations being sought by decomposing the images into a number of frequency bands, and a new image being back-transformed again exclusively from the frequency-dependent correlations. As a result, the uncorrelated noise components of the image are left behind, and the newly produced retransformed image includes only correlated image components, that is to say components that are to be ascribed to actual object structures.